1972 Vacuum System

[Mike Files]

I have re-established the vacuum system/components to factory specs. The headlight doors operate perfect. The wiper door opens when the wiper switch is moved to the on position but, mid-way through the first cycle of the wipers, the door closes. The door then has to be opened manualy to let the wipers retract. Actuator and actuator relay have been checked for proper operation and vacuum leaks as well as all hoses.
Thanks for any help.
Mike

[Joe L.]

Re: 1972 Vacuum System

Mike-----

I can tell you this: my original owner 1969 has exhibited the symptom that you describe from the day it was new. I really think that if the system operates without this foible, then the car is non-original (i.e. someone has taken the time to diagnose and correct it and achieved a state of operation which St. Louis rarely did).

[Jim T.]

Re: 1972 Vacuum System

Mike until you find the problem you can always raise the wiper door with the pull down switch under the steering column and use your wipers. The pull down switch is handy for using the wipers only when needed and quickly without having to wait for the wiper door to open.

[Paul O.]

Re: 1972 Vacuum System

Mike did you check the wiper arm park vacuum switch under the wiper door right side. If it is not porting vacuum correctly it could signal the relay to close the door at any time. When the door moves to closed it also will remove elec power from the wiper motor via the firewall power switch. Hope this helps. Paul #18046

[Jack H.]

One More Thing....

The vac system is intentionally 'biased' based on the relative diameter of the hoses used to port vac for 'control' vs. 'servo' functions. The objective is to force the control side of the vac system to come first such that when vac begins to flow through the servo side, the position of various vac motors is firmly established (open vs. closed).

When there are system leak(s), absolute vac pressure can fall to the point where 'race' conditions exist (servo side has power to actuate but control side is indefinte). This results in 'winking' headlight door(s) and 'dancing' wiper door.

You can also duplicate this instability without having leak(s) in the system. The check valve can be faulty (leaking) causing the vac reservior to hold insufficient reserve vacuum. Hence, after a given vac motor has consumed vac doing it's 'thing' on the servo side, there's insufficient reserve pressure to maintain the control side of the system and things 'hiccup' instead of opening and staying opened.

The other way to trigger the same symptom, is to have a clogged in-line vac filter. Here, the filter element is so dirty it's acting as a reducing agent to the absolute flow of vac from the intake, through the check valve, to the vac storage tank. The same thing happens....once a major servo element has 'fired' there's insufficient reserve vac to retain settings on the control side of the system....

The filter is often overlooked. It's a routine maintenance item that needs to be replaced periodically. Diagnose it two ways:

(1) Remove and hold the filter to the light using your eyeball to sight through the passage nipples. You should see clean, white, cotton. If it's black as a coal bin inside, get a new filter!

(2) With the engine running pulling steady state vacuum, disconnect the hoses to the filter and feel with your finger/thumb the vac pressure on both sides of the filter. If the inlet side (toward the intake) is MUCH higher/stronger than the outlet nipple, it's time to replace the vac filter....

[Mike Files]

Re: 1972 Vacuum System

I replaced the control valve/park vacuum switch also. Also, during a recheck of hose routing, I accidently reversed the two hoses connecting to the control valve/park switch. The door would then remain open but would not close when the wipers retracted. I could 'open' the by-pass valve under the dash and it would close. Which should be backwards. Thanks for your help.
Mike

[Mike Files]

Re: One More Thing....

Thank you for your help. I hadn't thought of checking the check valve or filter. Will do so this weekend.

[Mike Files]

Re: 1972 Vacuum System

Thanks Joe. I had a 68 roadster that was prone to eratic wiper door operation, but it would stay open, most of the time.